Steam turbines having such an intermediate steam extraction outlet are known. They are utilized in many different applications, such as chemical industry or process industry, where there is a need for steam of a determined pressure or temperature. One such example is an integrated paper mill where steam of relatively high temperature is needed for the pulping process and steam of a relatively lower temperature is needed for drying the paper.
In order to control the extraction of steam, it is known to provide a valve mechanism. Thereby, one may differ between internally controlled extraction and externally controlled extraction. Whereas externally controlled extraction merely includes a valve mechanism provided on the extraction outlet conduit, internally controlled extraction involves a valve mechanism within the casing of the steam turbine. A number of different solutions have been proposed in the past.
According to one solution, the steam from a first turbine part is guided radially outwards and collected in chambers that are disposed radially outside the normal casing of the turbine. The non-extracted steam is, by means of a valve arrangement guided along a complicated flow path radially inwards to the inlet of a second turbine part. This solution is very bulky and the flow of the steam is disturbed, resulting in relatively high pressure losses.
According to another solution, a rotatable disk having a number of openings is provided downstream of a steam extraction outlet but upstream of a partition wall having a corresponding number of openings. By rotating the disk the disk openings may be aligned with the partition wall openings, thus permitting a steam flow from a first turbine part to a second turbine part. The disadvantages of this solution are that the rotation of the disk requires forces which may be difficult to attain when the pressure of the steam is high, and that the possible opening area is limited to half the inlet area of the turbine part that is downstream of the partition wall. Also, in this solution, the steam flow is disturbed, resulting in relatively high losses of pressure.
According to still another solution, a partition wall is provided in a plane to which the longitudinal axis of the turbine forms the normal and downstream a steam extraction outlet. The wall has a number of passages provided with valve disks disposed against a valve seat in a plane perpendicular to the first mentioned plane. The valve disks may be lifted from the seats along a tangential direction. Also this arrangement results in a complicated flow path involving several changes of the steam flow direction.